1. Field of the Invention
The present invention relates generally to an optical element, and more particularly to an optical system and an exposure apparatus which has the optical element.
2. Description of the Related Art
A projection exposure apparatus configured to expose a pattern of an original (e.g., a reticle or a mask) on a substrate such as a wafer by using a projection optical system has been conventionally used, and immersion exposure has recently called attentions because it is one means for meeting the high-resolution demand. Since a numerical aperture (“NA”) of a projection optical system is defined as NA=n×sin θ, where “n” is a refractive index of a medium on the substrate side, the NA can be increased up to a value equal to “n” by filling a gap between the projection optical system and the substrate with a medium having a refractive index greater than that of air (n>1). By using this approach, the immersion exposure attempts to reduce the resolution R ( ) of the exposure apparatus, which is defined as R=k1×(λ/NA) where k1 is a process constant, and λ is a wavelength of the light source.
In the recent immersion exposure, a development of a medium having a higher refractive index is sought. Some liquids are known which have refractive indices of 1.64±0.01 for use with a laser light from an ArF excimer laser having a wavelength of 193 nm. See Roger H. French, et al., “Second generation fluids for 193 nm immersion lithography,” Optical Microlithography XIX, edited by Donis G. Flagello, Proc. of SPIE Vol. 6154, 615415, (2006), Yong Wang, et al., “High-Refractive-Index Fluids for the Next Generation ArF Immersion Lithography,” Advances in Resist Technology and Processing XXII, edited by Qinghuang Lin, Proc of SPIE, Vol. 6153, 61530A, (2006), Julius Santillan, et al., “Novel High Refractive Index Fluids for 193-nm immersion lithography,” Optical Microlithography XIX, edited by Donis G. Flagello, Proc. of SPIE Vol. 6154, 61544Q, (2006). The LuAg substrate having a refractive index of about 2.14 is also known as a glass material for a final lens in the projection optical system for use with the above high refractive index liquids. See John H. Burnett, et al., “High-index optical materials for 193 nm immersion lithography,” Optical Microlithography XIX, edited by Donis G. Flagello, Proc. of SPIE vol. 6154, 615418, (2006). It is also known that a multilayer antireflection film which includes, in order from a lens base side, an Al2O3 film, a SiO2 film, an Al2O3 film, and a SiO2 film that are arranged in order of the distance from the lens base side, may be used when the medium is pure water. See claim 7 and Paragraph 0061 of Japanese Patent Laid-Open No. (“JP”) 2006-179759. For a descriptive purpose, this application will let “193 nm” be the wavelength of the laser light from the ArF excimer laser although it is 193.4 nm, strictly speaking.
However, the antireflection film disclosed in claim 7 and paragraph 0061 of JP 2006-179759 may not be used with the high refractive index liquids disclosed in “second generation fluids for 193 nm immersion lithography,”, “High-refractive-Index Fluids for the Next Generation ArF Immersion Lithography,” and “Novel High Refractive Index Fluids for 193-nm immersion lithography,” and cannot limit reflectances for an incident angle range between 0° and 70° to 1% or less, in the high refractive index liquid. Applying the antireflection film disclosed in claim 7 and paragraph 0061 of JP 2006-179759 to the high refractive index liquid disclosed in “second generation fluids for 193 nm immersion lithography,” “High-refractive-Index Fluids for the Next Generation ArF Immersion Lithography,” and “Novel High Refractive Index Fluids for 193-nm immersion lithography” will cause flare or ghost and worsen the imaging performance.